This invention relates to the art of synthesizing diaminobenzenediols and related compounds.
Metal catalyzed hydrogenation of halo-dinitro-dihydroxy benzenes such as 2-halo-4,6-dinitroresorcinol is known to be useful for synthesizing diaminobenzenediols, such as 4,6-diaminoresorcinol. See Lysenko, High Purity Process for the Preparation of 4,6-Diamino-1,3-benzenediol, U.S. Pat. No. 4,766,244 (Aug. 23, 1988) which is incorporated herein by reference. During hydrogenation, the nitro groups are converted to amine groups, and the halogen is removed from the aromatic ring as hydrogen halide.
At high pH, the selectivity of the reaction is diminished. The patent of Liu, U.S. Pat. No. 5,001,265 (Mar.19, 1991), which is incorporated herein by reference, shows that the reduction can be carried out in an aqueous solution that contains an acid that is weaker than a hydrogen halide but is strong enough to protonate and protect the aminophenol. That process is advantageous because it uses an aqueous solvent, but it has other drawbacks. It produces an aqueous waste stream that contains both hydrogen halide ions (from the halogen that is removed during hydrogenation) and other ions (such as phosphate ions) from the protective acid. Such mixed waste streams are more difficult to purify or recycle than streams containing a single ion.
What is needed is a method to reduce the halo-dinitro-benzenediol compound to an aromatic diaminodiol rapidly with high selectivity without putting a mixture of different ions into the waste stream.